The review, which adds 27 studies to update a previous Cochrane review, confirms earlier analyses by "providing definitive evidence that pharmaceutical industry funding of drug studies biases the results and conclusions to look favourable towards the drug of the sponsor," said senior author, Professor Lisa Bero of the University of Sydney's Charles Perkins Centre.

The authors note there are several potential ways that industry sponsors can influence the outcome of a study, including the framing of questions, the design of a study, the conduct of a study, how data are analysed, selective reporting of favourable results, and "spin" in reporting conclusions.

Also, while some journals now require that the role of the sponsor in the design, conduct and publication of the study be described, this practice is not widespread.

Professor Bero said the key concern associated with industry-sponsored research evaluating drugs and medical devices was that there were no standard tools or validated criteria that include industry sponsorship as a risk of bias for such studies.

"We need bias assessments tools for drug studies that take funding source into account," Professor Bero said. "Currently, we have no validated way to detect or evaluate these subtle but systematic biases."

Co-author to the review, Dr Joel Lexchin, Professor Emeritus of York University, said the findings were especially concerning for patients and doctors.

"Our views about the effectiveness and safety of many medicines may be distorted. Medicines may be both less safe and less effective than we think to the extent that the evidence about them comes from the companies making them," he said.

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